Color takes on different appearances under different viewing conditions in such as the illumination white point, the illumination brightness, the ambient light level and the like. While the von Kries chromatic adaptation model in the known art is often adopted when executing a chromatic adaptation transform by taking into consideration the difference between the white points, the CIECAM02 may be adopted as a color appearance model if further details in the difference between the viewing conditions, including the difference in the luminance and the difference in the ambient light level, must be factored in. Since visual perception is not always completely adapted to the illumination condition provided in a given viewing condition, a chromatic adaptation transform that takes into consideration adaptation factors D (0.0≦D≦1.0) each indicating the rate of adaptation to a given viewing condition, the light source used under which a white point different from that of the light source used under a different viewing condition, is executed.
The adaptation factor D is determined in conformance to a specific viewing condition. For instance, in the color appearance model defined in the CIE, such as the CIECAM 97s or the CIECAM02, a value calculated based upon an adapted luminance LA (normally calculated as LA=Ldw/5 in relation to the illumination brightness level Ldw) and a parameter F determined in correspondence to the ambient light level (whether or not the environment is bright or dark) is used (patent reference 1).
                    [                  Expression          ⁢                                          ⁢          1                ]                                                            D        =                  F          ·                      [                          1              -                                                (                                      1                    3.6                                    )                                ⁢                                  ⅇ                                      (                                                                  -                                                  (                                                                                    L                              A                                                        +                            42                                                    )                                                                    92                                        )                                                                        ]                                              (        1        )                                          L          A                =                              L            dw                    /          5                                    (        2        )            
In addition, patent reference 2 proposes a method whereby the adaptation factor is adjusted based upon the relative color temperature of the illumination.    Patent reference 1: Japanese Patent Publication No. 3634633.    Patent reference 2: Japanese Laid Open Patent Publication No. H11-055688